Conventional OFDM-based Multiple Input, Multiple Output (MIMO) wireless data communication systems employ a feedback mechanism that provides information describing the condition of the OFDM channel so that transmitters on the channel can maximize the bandwidth of the channel. Each packet transmitted over the channel includes a preamble, a signal field, and a payload. Receivers use the preamble to synchronize with the transmitter, perform channel estimation, and adjust gain settings. The signal field specifies the data rate of the data packet that follows, the number of antennas used, and additional information to assist the receiver in decoding the data packet.
Each receiver on the channel measures the channel while receiving the preamble of each packet. The signal field includes a channel condition request bit that, when set, requests a channel condition report from the receiver. When the receiver receives a packet with the channel condition request bit set, the receiver transmits a channel condition report based on the most recent channel condition measurement taken by the receiver.
The receiver does not know in advance when it will receive a channel condition request. And by the time the receiver processes the signal field in a packet to determine whether the channel condition request bit in the signal field is set, the preamble portion of the packet has already passed through the receiver, and so cannot be measured. Therefore this scheme requires that the measurement circuit that measures the channel condition be constantly active (that is, powered). Therefore the measurement circuit constantly consumes power, which constitutes a significant battery drain for a mobile receiver.